The present invention relates to Schottky semiconductor devices and, more particularly, to power GaAs Schottky diodes and related semiconductor devices and methods of making such devices. The proposed Schottky semiconductor devices are particularly suited for application as high-voltage high-speed rectifiers.
Power Schottky diodes enjoy a number of advantages over P-N or P-i-N diodes. For example, the forward voltage drop, V.sub.F, of a Schottky barrier is significantly lower than that of a P-N junction and can be made as small as 0.5-0.6 V at high current densities. Furthermore, the current transport across a Schottky barrier is mediated by majority carriers, eliminating minority carrier injection and recombination, thereby making possible much higher switching rates.
A major limitation of conventional Schottky diodes is their large leakage currents relative to those of P-N junctions. This leakage puts an upper limit of about 100 V on the reverse blocking voltage of silicon-based Schottky diodes having an acceptable V.sub.F.
The properties of silicon Schottky diodes are limited by certain inherent material constraints, namely, low carrier mobility and a relatively narrow band gap. In designing a high-voltage diode, a thick low-doped base must be used, which leads to high series resistance, a bigger V.sub.F and higher heat dissipation. At higher temperatures the reverse leakage current, I.sub.R, increases dramatically, rendering the rectification ineffective.
The power Schottky diode, primarily silicon-based, has found extensive usage in high frequency rectifier circuits and power supplies because of its high switching rate and low power loss. However, the use of Schottky diodes made on silicon is limited in many applications by a relatively low voltage, usually less than 70 V, and high leakage current, especially at elevated temperatures.
In an effort to get around the limitations offered by silicon, several alternative materials have been studied, including diamond and SiC. Attention has lately been largely focused on gallium arsenide. It has been shown (see, B. J. Baliga, "Modem power devices" John Wiley & Sons, (1987); G. Ashkinazi, Tz. Hadas, B. Meyler, M. Nathan, L. Zolotarevski and O.Zolotarevski, Nickel-gallium arsenide high-voltage Schottky diodes, Solid State Electron., vol. 36, pp.13-18 (1993), which are incorporated by reference as if fully set forth herein) that several of the disadvantages associated with silicon Schottky diodes my be overcome by use of a gallium arsenide (GaAs) N.sup.+ N.sup.0 structure.
GaAs displays a higher electron mobility, a wider band gap and a higher peak electrical field strength at breakdown than silicon. These bulk properties make possible the design of a Schottky diode having a thinner base region, which can operate with greater forward current density at higher temperatures, has a small forward voltage drop, V.sub.F, and lower leakage current density, j.sub.R.
Relatively few power GaAs Schottky diode studies have been reported to date. The practical implementation of power GaAs Schottky diodes is inhibited by the difficulties encountered in producing a near-ideal barrier over a large area. A near-ideal barrier would have an ideality factor of nearly 1.02 and would have a very low density of interface states, or none at all.
High-voltage power Schottky diodes on a GaAs substrate having reverse voltages of up to 200-400 V have been reported (see, B. J. Baliga above, and B. J. Baliga, A. R. Sears, M. M. Barnickle, P. M. Campbell, W. Garwacki and J. P. Walden, IEEE Trans. Electron Devices, ED-32, 1130, (1985); K. Ohtsuka, Y. Kusuzawa, K. Ogata, H. Ichinosawa, U.S. Pat. No. 5,027,166, (1991), which are incorporated by reference as if fully set forth herein) but the fabrication of these diodes requires the use of ultra-high vacuum techniques and/or other expensive methods of preparation, such as molecular beam epitaxy. These techniques are employed since it is imperative to obtain a metal-semiconductor rectifying barrier without any residual oxide and with minimal surface states density at the metal-semiconductor interface.
A simple and inexpensive technology for the fabrication of near-ideal Schottky barriers over large areas has been reported (G. Ashkinazi, Yu. Zilyaev, V. Chelnokov and M. Shul'ga, Sov. Techn. Phys. Lett. (USA), 9, 177 (1983); G. Ashkinazi, L. Zolotarevski, L. Mazo, V. Timofeev, M. Shul'ga and V. Shumilin, Proc. 3rd European Conf. on Power Electron. and Appl., Aachen, FRG, 617 (1989), which are incorporated by reference as if fully set forth herein). The technology involves the chemical deposition of nickel on epitaxial GaAs N.sup.+ N.sup.0 structures. The first laboratory devices were reported in 1983 and improvements, which resulted in Schottky diodes with an operating V.sub.BR of up to 70 V and forward currents of up to 20 to 40A, were reported in 1989.
Another important problem involves minimizing the current of the reverse biased diode, or leakage current, under high reverse voltage. The leakage current is made up of three main components: (a) the reverse current through an ideal barrier; (b) the leakage current through barrier defects, and (c) the leakage current determined by an enhanced electric field on the surface, and other phenomena connected with the surface condition.
The reverse current through an ideal barrier cannot be altered for a given set of barrier parameters. The leakage current through barrier defects can be reduced by using a high quality barrier and a high quality epitaxial layer. To overcome the leakage current determined by an enhanced electric field on the surface, various means are used, including metal field plates, guard rings, and combinations of these (see, B. J. Baliga (1987) above, and S. M. Sze "Physics of semiconductor devices", (1986), which is incorporated by reference as if fully set forth herein), or semi-insulating protection electrodes (see K. Ohtsuka et al. above). A second set of possibilities includes the use of films and/or dielectric compounds and resins.
The first group of methods referenced above requires the use of expensive deposition, photolithography and lift-off apparatus, and also decreases the switching ability of a Schottky diode, while the latter group of techniques requires specially developed side profiling, treatment and protection technology.
There is thus a widely recognized need for, and it would be highly advantageous to have, high-voltage high-speed gallium arsenide power Schottky semiconductor devices, such as diodes, and methods of fabricating them, which devices would be simple and inexpensive to fabricate and which would have performance characteristics significantly superior to those exhibited by corresponding silicon devices.